Superplastic aluminum alloy and process of producing same

ABSTRACT

The present invention provides a superplastic aluminum alloy in which fine particles not substantially dispersion hardening are dispersed in a sufficient amount to effect grain boundary pinning to suppress crystal grain growth during hot working thereby ensuring manifestation of superplasticity over wide ranges of working temperature and strain rate.  
     According to the present invention, a superplastic aluminum alloy contains ceramic particles having an average particle size of from 10 nm to 500 nm in an amount of from 0.1 vol% to 5 vol% and a process of producing a superplastic aluminum alloy, comprises the steps of: hot working an aluminum alloy ingot containing ceramic particles having an average particle size of from 10 nm to 500 nm in an amount of from 0.1 vol% to 5 vol% with a working degree of from 10% to 40% at a temperature of 400° C. or higher; heat-treating at a temperature of 400° C. or higher; and hot-working with a working degree of 40% or more at a temperature of lower than 400° C.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a superplastic aluminum alloyand a process of producing the same.

[0003] 2. Description of the Related Art

[0004] The hot-workability of aluminum alloy has been improved byutilizing the giant elongation and low resistance to deformation in thesuperplastic state.

[0005] However, in the conventional art, superplasticity can be onlyutilized in a thin sheet having a uniform deformation becausesuperplasticity is only manifest at certain temperatures and at certainstrain rates.

[0006] If superplasticity can be manifested over a wide range oftemperatures and strain rates, superplasticity can be utilized forextrusion and forging processes in which temperatures and strain ratesare different between portions subject to deformation.

[0007] To manifest superplasticity in wide ranges of temperature andstrain rate, it is important to control the change in microstructure,particularly the growth of crystal grains. To this end, it is necessarythat the aluminum alloy contains large amount of fine particles whichpin the grain boundary migration.

[0008] Metal particles and ceramic particles have been used as particleseffectively pinning the grain boundaries.

[0009] To introduce metal particles in an aluminum alloy, precipitationin a solid phase and crystallization from a liquid phase are possible.In the precipitation, a large amount of metal elements must be in solidsolution to precipitate a large amount of metal particles sufficient tosuppress crystal grain growth. In usual ingot process, the solid solubleamount is limited, so that metal elements cannot be brought into solidsolution in an amount sufficient to precipitate a large amount ofparticles necessary to effect pinning of gain boundaries. JapaneseUnexamined Patent Publication (Kokai) No. 3-28344 proposed a method offorcibly making a solid solution by powder metallurgy. However, powdermetallurgy has a problem in that the production cost is high and thematerial shape is also limited.

[0010] On the other hand, in the crystallization, it is important thatparticles are finely and uniformly crystallized from a melt. To thisend, Japanese Unexamined Patent Publication (Kokai) No. 8-74012 proposeda method of reacting an aluminum melt with a ceramic powder to formmetal particles. However, this method requires a long time for thereaction and the reaction is difficult to control.

[0011] To introduce ceramic particles into an aluminum alloy, thefollowing two methods are proposed. In the first method disclosed inJapanese Unexamined Patent Publication (Kokai) No. 8-74012, metalelements in an aluminum melt are reacted with a blown-in gas to formceramic particles. However, as described above, this method requires along time for the reaction and the reaction is difficult to control. Thesecond method is to add ceramic particles to an aluminum melt. However,this method has a problem that it is generally difficult to uniformlydisperse ceramic particles in an aluminum alloy melt. JapaneseUnexamined Patent Publication (Kokai) No. 6-235032 proposed a method inwhich a ceramic powder is uniformly mixed with an aluminum alloy powderand is then press-formed. However, this method not only causes anincrease in the production cost but also limits the material shape.Moreover, this method uses coarse TiC particles having a particle sizeup to 45 μm, which increases the dispersion strengthening effect whenpresent in a large amount, so that the high temperature strength isincreased to render the thermomechanical treatment difficult and theroom temperature strength is also increased as well as the elongation isreduced to render the secondary operation after superplastic deformationdifficult. Therefore, particles cannot be introduced in an amounteffective for grain boundary pinning.

SUMMARY OF THE INVENTION

[0012] The object of the present invention is to provide a superplasticaluminum alloy in which the above-mentioned conventional problems aresolved by dispersing fine particles having no substantial dispersionstrengthening effect in an amount sufficient to effect grain boundarypinning to suppress grain growth during hot working, thereby enablingmanifestation of superplasticity in a wide range of temperatures andstrain rates.

[0013] To achieve the object according to the present invention, thereis provided a superplastic aluminum alloy containing ceramic particleshaving an average particle size of from 10 nm to 500 nm in an amount offrom 0.1 vol% to 5 vol%.

[0014] In the present invention, the term “superplasticity” means thatan elongation of 200% or more is obtained in a high temperature tensiletest at a hot working temperature or a temperature of Tm/2 or higher, Tmbeing a melting point expressed in terms of absolute temperature.

[0015] The aluminum alloy of the present invention is an alloy composedof a matrix containing a dispersion of ceramic particles. The matrix maybe composed either of aluminum alone or an aluminum alloy containing Si,Cu, Mn, Mg, Cr, Zn or other elements which are usual alloying elementsof aluminum alloys. The alloy may further contain one or more of REM,Zr, V, W, Ti, Ni, Nb, Ca, Co, Mo, and Ta which form globular particlesof intermetallic compounds with aluminum during homogenizationtreatment. These elements may be contained in an amount at which nogiant crystallized particles are formed. However, these elements arepreferably contained in a small amount when ceramic particles arepresent in a large amount which renders thermomechanical treatment orsecondary operation difficult. Intermetallic compounds preferably have aparticle size of 500 nm or less, the same as for ceramic particles.

[0016] Fe or other impurities usually present in aluminum alloys areacceptable if they are present in an amount within the range in which nogiant crystallized particles are formed.

[0017] Ceramic particles may be acceptable if they do not react with thematrix aluminum and alloying elements and if stable during hot workingand may be carbides, nitrides, carbonitrides, borides, silicides,oxides, and so on. The aluminum alloy of the present invention maycontain particles of one or more ceramics, Ceramic particles may beintroduced in an aluminum alloy by any method, such as in-situ processesin an aluminum melt, vapor phase processes, solid phase processes,firing of metal complexes, and so on, although in-situ processes aremost preferably because of good wettability with aluminum melt anduniform dispersion. The shape of the ceramic particles are not limitedso long as a required pinning is achieved while dispersion strengtheningis avoided, although a spherical shape is most preferred from aviewpoint of formability.

[0018] Grain boundary pinning is not achieved if the ceramic particleshave too small or large a size. To ensure grain boundary pinning,ceramic particles must have an average size of from 10 nm to 500 nm.

[0019] When the average particle size is less than 10 nm, grain boundarypinning is not achieved because dislocation cells are difficult to formbecause the dislocations introduced during hot working form loops, etc.When the average particle size is more than 500 nm, dislocation cellsare difficult to form and grain boundary pinning is not achieved.Average particle size of more then 500 nm are also disadvantageousbecause dispersion strengthening is significant to increase the hightemperature strength failing to provide superplasticity and also toincrease the room temperature strength and reduce the elongation torender the secondary operation difficult. It should be noted that whenthe average particle size is more than 300 nm, grain boundary pinning isnot significantly increased. Therefore, to ensure grain boundary pinningwhile avoiding dispersion strengthening, the average particle size isnot preferably more than 300 nm.

[0020] The content of ceramic particles must be 0.1 vol% or more butmust not be more than 5 vol% to avoid dispersion strengthening. Itshould be noted that if the content is more than 1 vol%, grain boundarypinning is not significantly increased. Therefore, to ensure grainboundary pinning while avoiding dispersion strengthening, the content ofceramic particles is not preferably more than 1 vol%.

[0021] The ceramic particles are preferably dispersed at an averageinterparticle distance of not more than 50 μm to promote grainrefinement.

[0022] In a preferred embodiment, an aluminum alloy according to thepresent invention contains Mg in an amount of 4 wt % or more. Mg is amain element for strengthening through a strengthening mechanism inwhich resistance to transgranular deformation is increased by solidsolution strengthening and a decrease in cross slip due to reduction instacking fault energy. This decreases the grain boundary strength withrespect to the grain strength at high temperatures to promote grainboundary migration and grain boundary slip, thereby facilitatingmanifestation of superplasticity. This effect is significant when the Mgcontent is 4 wt % or more. The upper limit of the Mg content is notnecessarily specified but when the content exceeds 15 wt %, the hotworkability is too low to be practically acceptable. Cu, Zn or elementshaving grain strengthening effect because of reduction in stacking faultenergy are also utilized to promote superplasticity in the samemechanism as for Mg.

[0023] Aluminum alloys containing Mg as a main alloying element are alsoadvantageous because the room temperature elongation is large tofacilitate secondary operation after superplastic deformation andbecause both elongation and strength are high to provide good toughness.Conventionally, aluminum alloys containing Mg in an amount of 2 wt % ormore had poor hot workability and were difficult to extrude or forge.According to the present invention, it is possible to perform hotsuperplastic forming of high strength, high toughness aluminum alloyscontaining Mg in an amount of 4 wt % or more.

[0024] A process of producing a superplastic aluminum alloy according tothe present invention comprises the steps of: hot working an aluminumalloy ingot containing ceramic particles having an average particle sizeof from 10 nm to 500 nm in an amount of from 0.1 vol% to 5 vol% with aworking degree of from 10% to 40% at a temperature of 400° C. or higher;heat-treating at a temperature of 400° C. or higher; and hot-workingwith a working degree of 40% or more at a temperature of lower than 400°C.

[0025] The first hot working destroys a cast structure. If the caststructure remains, the second hot working cannot form a uniform finestructure of dislocation cells necessary to provide grain boundarypinning. The first hot working is performed at a temperature of 400° C.or higher to avoid precipitation of solute atoms and impurities.However, the temperature must not be higher than the solidus line toprevent formation of a liquid phase. Temperatures of from 400° C. to500° C. are generally suitable.

[0026] To destroy the cast structure, the working degree must be atleast 10%. The upper limit of the working degree is 40%, because theeffect does not increase at greater working degrees.

[0027] The first hot working provides a nonuniform worked structure(dislocation structure) which, if it remained unchanged, would cause anonuniform dislocation cell structure to be formed by the second hotworking and the desired fine grained structure would not be achieved.Therefore, a heat treatment is performed after the first hot working toextinguish the nonuniform worked structure.

[0028] Heat treatment temperatures of lower than 400° C. require a longtreatment time and are not practically acceptable. On the other hand,the heat treatment temperature must not be higher than the solidus lineto prevent formation of a liquid phase. Temperatures of from 400° C. to500° C. are generally suitable. The heat treatment time is suitably from1 to 4 hours.

[0029] The heat treatment may either be performed immediately after thefirst hot working without cooling or may be performed by reheating aftercooling to the ambient or room temperature.

[0030] The second hot working introduces dislocations entangled withuniformly distributed particles to form equiaxed dislocation cells,thereby providing an equiaxed fine grained structure to manifestsuperplasticity.

[0031] If the second hot working is performed at a temperature of 400°C. or higher, recovery of dislocation occurs during the working and thedesired fine grained structure is not achieved. The lower limit of thetemperature is selected to prevent cracking from occurring during theworking. Temperatures of from 200° C. to 300° C. are generally suitable.

[0032] To provide a fine grained structure necessary to manifestsuperplasticity, the working degree must be at least 40%.

[0033] The second hot working may either be performed subsequent to theheat treatment after cooling to a temperature of lower than 400° C. ormay be performed by reheating after cooling to the ambient or roomtemperature.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034]FIG. 1 is a graph showing the elongation observed in temperaturetensile test of a superplastic aluminum alloy according to the presentinvention as a function of the testing temperature and the strain rate;

[0035]FIG. 2 is a graph showing the elongation observed in hightemperature tensile test of a conventional superplastic aluminum alloyas a function of the testing temperature and the strain rate; and

[0036] FIGS. 3(1) and 3(2) are microphotographs of (1) an aluminum alloyaccording to the present invention and (2) a conventional aluminum alloyafter high temperature tensile test.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0037] The present invention will be described in further details by wayof examples below.

EXAMPLE 1

[0038] Aluminum alloy ingots having different chemical compositionssummarized in Table 1 were homogenization heat-treated at 440° C. for 24hours.

[0039] The ingots were then swaged, as the first hot working, at 400° C.with a working degree of 10%, and immediately thereafter, heat-treatedat 400° C. for 1 hour followed by water cooling. The second hot workingwas then performed by hot swaging at 300° C. with a working degree of50%, followed by water cooling.

[0040] Test pieces having a 5 mm dia., 10 mm long gauge portion were cutfrom the thus-produced samples and subjected to tensile test attemperatures of from 300° C. to 500° C., strain rates of from 1.7×10⁻⁴/sto 1.7×10⁻¹/s. The results are summarized in Table 1.

[0041] Sample No. 1 according to the present invention contained 0.2vol% of TiC particles having an average diameter of 200 nm, which wereproduced through in-situ process by reacting stoichiometric proportionsof Ti and C in a pure aluminum melt. Superplasticity (elongation of 200%or more) was observed over wide ranges of working temperatures andstrain rates as can be seen from FIG. 1.

[0042] Comparative sample No. 2 containing no particles did not exhibitsuperplasticity.

[0043] Comparative samples No. 3 and No. 4 were conventionalsuperplastic aluminum alloys and contained dispersion strengtheningparticles of Al₃Zr, which is not a ceramic but an intermetalliccompound, so that superplasticity was manifested in narrower ranges ofworking temperature and strain rate. This is shown in FIG. 2 for sampleNo. 4.

[0044] After the high temperature tensile test, the test pieces weresubjected to optical microscopic observation for the internal structureand scanning electron microscopic observation for the surface structure.FIGS. 3(1) and 3(2) show typical examples.

[0045] Sample No. 1 containing dispersed ceramic particles having a sizeand a content according to the present invention and exhibiting asuperplasticity of up to 300% contained extremely small amount ofcavitation or voids as shown in FIG. 3(1), which ensures good roomtemperature strength and secondary operability.

[0046] Comparative sample No. 4 containing dispersed intermetalliccompound particles contained large amount of cavitation as shown in FIG.3(2), which causes significant degradation of room temperature strengthand secondary operability even if the sample was deformed to one half ofthe superplastic elongation.

EXAMPLE 2

[0047] Aluminum alloy ingots having different chemical compositionssummarized in Table 2 were thermomechanically treated as in Example 1.

[0048] Sample No. 5 according to the present invention contained in-situproduced TiC particles having an average particle size of 200 nm in anamount of 0.7 vol% as in Sample 1 of Example 1. Comparative Samples 6 to8 have the same chemical compositions as those of Comparative Samples 2to 4 of Example 1, respectively.

[0049] From the thus-produced samples, extrusion test pieces having adiameter of 7 mm and a length of 10.5 mm were cut and subjected toextrusion test with an extrusion ratio of 14, at a temperature of 400 to500° C., and a strain rate of 3.5×10⁻²/s to 3.5 ×10⁰/s. The results aresummarized in Table 3. The extrudability was evaluated such that whenthe extrusion stress was less than that required for JIS 7003 alloy, thetested sample had a better extrudability and when a more extrusionstress was required, the tested sample had a poor extrudability.

[0050] Sample 5 of the present invention exhibited a good extrudability,i.e., required a low extrusion stress.

[0051] Comparative Sample 5 contained no dispersed particles andexhibited a very poor extrudability because of a very high extrusionstress required.

[0052] Comparative Samples 7 and 8 did not exhibit superplasticity overwide ranges of temperatures and strains rate and required high extrusionstress causing poor extrudability.

EXAMPLE 3

[0053] Aluminum alloy ingots having the same chemical composition asthat of Sample 1 of Table 1 were thermomechanically treated underdifferent conditions summarized in Table 4.

[0054] Test pieces having a 5 mm dia., 10 mm long gauge portion were cutfrom the thus-produced samples and subjected to tensile test attemperatures of from 300° C. to 500° C., and at strain rates of from1.7×10⁻⁴/s to 1.7×10⁻¹/s. The results are summarized in Table 4.

[0055] Sample 9 of the present invention exhibited superplasticity overwide ranges of temperatures and strain rates.

[0056] Comparative Sample 10 was not homogenization-treated andcontained undissolved giant crystallized substances, so that the secondhot working failed to form uniform fine-grained structure andsuperplasticity was not manifested.

[0057] In Comparative Sample 11, the first hot working was performed ata small working degree and the cast structure was not completelydestroyed, so that the second hot working failed to form uniformfine-grained structure and superplasticity was not manifested.

[0058] In Comparative Sample 12, the first hot working was performed ata lower temperature to form coarse acicular precipitates of impurities,so that the second hot working failed to form uniform fine-grainedstructure and superplasticity was not manifested.

[0059] In Comparative Sample 13, the heat treatment after the first hotworking was performed at a low temperature and nonuniform workedstructure retained, so that the second hot working failed to formuniform fine-grained structure and superplasticity was not manifested.

[0060] In Comparative Sample 14, the second hot working was performed ata high temperature and failed to form uniform fine-grained structure, sothat superplasticity was not manifested.

[0061] In Comparative Sample 15, the second hot working was performedwith a small working degree and failed to form uniform fine-grainedstructure, so that superplasticity was not manifested. TABLE 1 Volumepercentage Chemical composition [wt %] of dispersed Manifestation ofsuperplasticity Mg Ti C Zr Fe Si Al particles [vol %] Temperature [° C.]Strain rate [S⁻¹] Invention No. 1 4.99 0.34 0.05 — 0.02 0.01 Bal. TiC:0.2 400-500 1.7 × 10⁻³ - 10⁻¹ Comparison No. 2 5.02 — — — 0.02 0.01 Bal.— — — No. 3 4.63 — — 0.21 0.02 0.01 Bal. Al₃Zr: 0.25 500 1.7 × 10⁻¹ No.4 9.21 — — 0.12 0.02 0.01 Bal. Al₃Zr: 0.14 400-420 1.7 × 10⁻² - 10⁻¹

[0062] TABLE 2 Volume percentage Chemical composition [wt %] ofdispersed Mg Ti C Zr Fe Si Al particles [vol %] Invention No. 5 4.841.21 0.28 — 0.02 0.01 Bal. TiC: 0.7 Comparison No. 6 5.02 — — — 0.020.01 Bal. — No. 7 4.63 — — 0.21 0.02 0.01 Bal. Al₃Zr: 0.25 No. 8 9.21 —— 0.12 0.02 0.01 Bal. Al₃Zr: 0.14

[0063] TABLE 3 Extrusion temperature Extrusion stress [MPa] [° C.] 400450 500 Extruda- Extrusion speed [S⁻¹] 3.5 × 10⁻² 3.5 × 10⁻¹ 3.5 × 10⁰3.5 × 10⁻² 3.5 × 10⁻¹ 3.5 × 10⁰ 3.5 × 10⁻² 3.5 × 10⁻¹ 3.5 × 10⁰ bilityInvention No. 5 100 198 242 68 102 179 46 94 134 Good Comparison No. 6222 271 330 130 205 269 85 147 219 Poor No. 7 163 244 326 115 170 246 77127 189 Poor No. 8 134 248 365 96 164 298 — — — Poor Reference 7003 93148 202 75 113 151 54 76 109 — material

[0064] TABLE 4 Intermediate High Temperature Homogenize 1st Hot WorkHeat Treatment 2nd Hot Work Elongation [%] Invention No. 9 440° C. × 24hr 400° C. × 10% 400° C. × 1 hr 300° C. × 50% 330 Comparison No. 10 —400° C. × 10% 400° C. × 1 hr 300° C. × 50%  70 No. 11 440° C. × 24 hr400° C. × 5% 400° C. × 1 hr 300° C. × 50% 110 No. 12 440° C. × 24 hr300° C. × 10% 400° C. × 1 hr 300° C. × 50% 145 No. 13 440° C. × 24 hr400° C. × 10% 300° C. × 1 hr 300° C. × 50% 160 No. 14 440° C. × 24 hr400° C. × 10% 400° C. × 1 hr 500° C. × 50% 150 No. 15 440° C. × 24 hr400° C. × 10% 400° C. × 1 hr 300° C. × 30% 175

[0065] As herein described above, the present invention provides asuperplastic aluminum alloy in which fine particles not substantiallydispersion hardening are dispersed in a sufficient amount to effectgrain boundary pinning to suppress crystal grain growth during hotworking thereby ensuring manifestation of superplasticity over wideranges of working temperatures and strain rates.

1. A superplastic aluminum alloy containing ceramic particles having anaverage particle size of from 10 nm to 500 nm in an amount of from 0.1vol% to 5 vol%.
 2. A superplastic aluminum alloy according to claim 1 ,which contains 4 wt % or more of Mg.
 3. A superplastic aluminum alloyaccording to claim 1 or 2 , wherein the ceramic particles are composedof TiC.
 4. A process of producing a superplastic aluminum alloy,comprising the steps of: hot working an aluminum alloy ingot containingceramic particles having an average particle size of from 10 nm to 500nm in an amount of from 0.1 vol% to 5 vol% with a working degree of from10% to 40% at a temperature of 400° C. or higher; heat-treating at atemperature of 400° C. or higher; and hot-working with a working degreeof 40% or more at a temperature lower than 400° C.